it's an idiom that means you work very hard. hmm... a familiar tag line in the song 'a hard day's night'. :)

Nope. This expression does not have anything to do with loyalty as dogs have often been exemplified to be. It just means to work very hard. " I worked like a dog to finish my assignment on time". There is a word called doggedly (adv) which also contain the part 'dog' and it has also a similar meaning.It means to work hard inspite of difficulties. Here again there isn't any connotations of loyalty. :)

Humm..not really sure.But i think it is to discribe someone who works loyally and deligently.

"work like a dog" means to work a lot but actually achieve little (for yourself). Can also say "work like a slave".